This section is intended to provide a background or context to the invention recited in the claims. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and claims in this application and is not admitted to be prior art by inclusion in this section.
Indium sulfide has long attracted attention as one of just a few simple sulfides with good stability, moderate band gap, and high carrier mobility. In2S3 has shown potential, in particular, in the photovoltaic industry as a buffer layer in thin film devices, replacing potentially toxic CdS layers. Indium(III) is also a principal component in some of the most efficient thin film chalcogenide PV alloys including CuInS2 and Cu(In,Ga)(S,Se)2. In addition to these supporting roles, In2S3 may serve as a well-behaved and photoactive absorber layer. Substitutionally doped alloys of In2S3 have further been identified as impurity-band absorbers with nearly ideal energy level separation for solar energy applications. A variety of deposition methods exist for In2S3 including chemical bath deposition, physical vapor deposition, ion layer gas reaction, sputtering, and atomic layer deposition (ALD). ALD is one of the most promising routes to optoelectronic materials as it enables digital control and conformal growth, sophisticated doping opportunities, as well as pinhole-free films of high density.
The self-limiting surface chemistries that constitute a well-defined ALD process afford unique synthetic control over interfaces, stoichiometry, and crystalline phase. Furthermore, many low-temperature ALD (<200° C.) processes have been identified that enable high quality materials growth in applications with limited thermal stability including flexible substrates or multilayers prone to diffusion. An important requirement for achieving electronic-quality materials through low-temperature ALD is the identification of a metal precursor with clean surface chemistry so as to exclude deleterious impurities. Though several reports of the ALD of In2S3 have been published, the processes leave something to be desired. Initial work with InCl3 showed good growth rates of 1.4 Å/cycle and lead to polycrystalline films. However, a volatilization temperature of at least 300° C. as required for InCl3 necessitates the use of more complex tooling including high temperature and chemically resistant valves. Composition analysis also revealed that optimized growth resulted in 3% Cl contamination, a value that is well beyond that acceptable for many applications. In an attempt to bypass high temperature halide-based processes, which produce strongly acidic byproducts known to etch films and damage tools, several reports have focused on indium (III) acetylacetonate (In(acac)3). This precursor allows for significantly lower growth temperatures and halide-free films when dosed alternately with H2S. However, a wide range of thin film properties result including a direct band gap of ˜2.8 eV, which is consistent with significant oxygen incorporation. The precursor is further limited by a modest thermal stability threshold reported to be 150° C. The resulting films exhibit non-ideal S/In ratios (1.42) and high carbon levels (3.7-5.1%), while the process suffers from low precursor volatility, and modest self-limiting growth rates −0.35 Å/cycle with demonstrated self-limiting behavior, though up to 0.7 Å/cycle has been reported—that all leave significant room for improvement. Until now there have been no reports of In2S3 thin films grown by ALD with an impurity level lower than 3%.
A need exists for improved technology, including technology that may address the above problems, namely by providing a method of oxygen-free atomic layer deposition of indium sulfide using a synthesized indium precursor and hydrogen sulfide.